A thermoplastic is a melt-processable polymeric material that, unlike a thermoset polymer, cannot be irreversibly cured through chemical crosslinking. A thermoplastic composite combines a thermoplastic polymer matrix with an embedded reinforcement phase such as a continuous filament or fibers. One notable example of a thermoplastic composite is a carbon-fiber-reinforced-polymer, or CFRP for short, which includes a thermoplastic polymer matrix reinforced with carbon fibers. A wide range of other thermoplastic composites are of course known in the art. Thermoplastic-based materials (e.g., thermoplastics or thermoplastic composites), in general, have some notable characteristics when compared to metals and thermoset materials, including their high strength-to-weight ratio, impact resistance, ability to withstand static and fatigue loads, potential for directional strength profiles, and the recyclable nature of the thermoplastic polymer matrix.
A number of industries—the automotive, aerospace, and marine industries in particular—have become interested in structural workpieces manufactured from thermoplastic-based materials as a possible substitute for metallic or thermoset-based workpieces. In the automotive industry, for example, vehicle body panels that include one or more thermoplastic-based workpieces are considered good candidates to achieve overall vehicle weight reduction and, in turn, improved fuel economy. When used in that way, a thermoplastic-based workpiece will typically have to be attached to a metal workpiece in a manufacturing setting during fabrication of the vehicle body panel. This presents a challenge as it can be difficult to attach thermoplastic-based workpieces to other types of workpieces, most notably metallic workpieces, by conventional techniques employed in a manufacturing setting at a reasonable cost.
Past attempts to try and attach a thermoplastic-based workpiece to a metal workpiece have yielded mixed results. Traditional fusing welding techniques—e.g., ultrasonic welding, resistance spot welding, etc.—typically require double-side access and are unable to obtain more than a weak joint due to the sharp difference in physical properties of the two workpieces. And while a particular laser welding technique known as laser-assisted metal and plastic joining (LAMP) has shown better results, conventional practices of the technique derive its joint strength primarily through mechanical interlock, which may not be suitable for all end-use applications. Still further, adhesive bonding is minimally effective since thermoplastic-based workpieces generally have low surface energy, often necessitating high-cost surface treatments like corona discharge or plasma treatments to be effective. Even the use of mechanical fasteners is challenging since they tend to concentrate stress in the workpiece and damage the reinforcing phase that may be present.
The current practice of attaching a thermoplastic-based workpiece to a metal workpiece employed by commercial manufactures involves the combined use of self-piercing rivets (SPR) and adhesive bonding. While this has yielded acceptable results in some instances, the overall process is complex, expensive, and time-consuming in a manufacturing setting. The combined use of adhesive bonding and self-piercing rivets also implicates the issues mentioned above; that is, the need to conduct surface treatments to improve the surface energy of the thermoplastic-based workpiece, the creation of stress concentrations around the self-piercing rivets, and the infliction of damage to the reinforcing phase if the thermoplastic-based workpiece is a thermoplastic composite workpiece. Self-piercing rivets, moreover, require double-sided access in order to complete the riveting process. In light of these challenges currently facing a number of manufacturing industries, there is an apparent need for a simpler and more effective way to attach a thermoplastic-based workpiece to a metal workpiece.